1. Field of the Invention
The present invention relates to a semiconductor device such as an insulated gate bipolar transistor (IGBT) having an insulated gate for controlling a current, and particularly, to a semiconductor device serving as a bidirectional switch that controls a direct current as well as an alternating current.
2. Description of the Prior Art
An insulated gate bipolar transistor (IGBT) is capable of controlling a large current in response to a relatively low bias voltage applied to the gate electrode thereof. The IGBT is a MOS bipolar composite function element adopted for power electronics, to control power and switch a current.
FIG. 1 is a circuit diagram showing a semiconductor switch employing an IGBT according to a prior art, and FIGS. 2A to 2D are sectional views showing the processes of fabricating the IGBT.
In FIG. 1, the semiconductor switch has the IGBT 103 having a collector connected to an output terminal 101, an emitter connected to an output terminal 102, and a gate electrode to be biased by a gate controller 104.
In FIG. 2A, an n.sup.- -type substrate 131 serving as an n.sup.- -type base layer is prepared. A p.sup.+ -type anode layer 132 is formed on the bottom surface of the substrate 131 by diffusing boron. Instead, a p.sup.+ -type substrate may be employed, and an n.sup.- -type epitaxial layer may be formed on the substrate. In this case, the substrate serves as a p.sup.+ -type anode layer 132, and the epitaxial layer serves as an n.sup.- -type base layer 131. In FIG. 2B, an SiO.sub.2 film 133 serving as a gate oxide film is formed on the top surface of the substrate 131, and a polysilicon layer 139 serving as an insulated gate electrode 136 is formed on the gate oxide film 133. A window is formed at the center of the polysilicon layer 139, and through the window, a p-type base layer 134 is formed by diffusing boron to the top surface of the substrate 131.
In FIG. 2C, the polysilicon layer 139 is left at a part to form the gate electrode 136, and a new SiO.sub.2 film 133 is formed on the top surface of the substrate 131. Two windows are formed at the center of the SiO.sub.2 film 133, and through the windows, an n.sup.+ -type emitter layer 135 is formed by diffusing, for example, arsenic to the base layer 134 by double diffusion.
The polysilicon layer 139 is patterned into the gate electrode 136 by photolithography or RIE. An SiO.sub.2 film is formed over the exposed base layer 134 and gate electrode 136. In FIG. 2D, an emitter electrode 137 is formed over the SiO.sub.2 film 133 at the center of the substrate 131. A collector electrode 138 is formed to cover the anode layer 132 on the bottom surface of the substrate 131, to complete the IGBT 103 of FIG. 1.
The operation of the IGBT will be explained. To turn ON the IGBT, the emitter electrode 137 is grounded, and a positive voltage is applied to the collector electrode 138. Under this state, a positive voltage is applied to the gate electrode 136 with respect to the emitter electrode 137. Similar to a MOSFET, the positive voltage applied to the gate electrode 136 forms an inverted channel, or inversion layer along the surface of the base layer 134, so that electrons are injected from the emitter layer 135 into the base layer 131 through the reverse channel. At the same time, holes are injected from the anode layer 132 into the base layer 131, to form a forward bias state in a pn junction between the anode layer 132 and the base layer 131. This changes the conductivity of the base layer 131, to turn ON the element. In this way, the IGBT is turned 0N when the conductivity of the high-resistance base layer 131 is changed to drastically decrease the resistance thereof. Even if the base layer 131 is low in impurity concentration and thick to increase a breakdown voltage, the ON-state resistance RON of the element will be very small. To turn OFF the IGBT, the gate electrode 136 is zero-biased with respect to the emitter electrode 137, or a negative voltage is applied to the gate electrode 136. Then, the reverse channel under the gate electrode 136 disappears, to stop the injection of electrons from the emitter layer 135. At this time, electrons are present in the base layer 131. Holes accumulated in the base layer 13i mostly flow into the emitter electrode 137 through the base layer 134 and partly recombine with the electrons in the base layer 131, to disappear. When all of the holes in the base layer 131 disappear, the element turns OFF.
The semiconductor switch of FIG. 1 employing the singular IGBT is capable of controlling only a current flowing from the output terminal 101 connected to the collector electrode 138 toward the output terminal 102 connected to the emitter electrode 137 and is incapable of controlling a current flowing reversely. Namely, this switch is incapable of controlling an alternating current. FIGS. 3 and 4 show conventional bidirectional semiconductor switches capable of controlling a direct current as well as an alternating current. The switch of FIG. 3 employs MOSFETs 113 and 114, which are connected oppositely in series between output terminals 111 and 112. The gates of the MOSFETs 113 and 114 are biased by a gate controller 115. This switch is capable of passing a current flowing from the output terminal 111 toward the output terminal 112, or from the output terminal 112 toward the output terminal 111. Namely, the switch controls a direct current as well as an alternating current.
The bidirectional semiconductor switch of FIG. 4 employs two IGBTs 123 and 124, which are connected oppositely in series between output terminals 121 and 122. A reverse diode 125 is connected between the collector and emitter of the IGBT 123, and a reverse diode 126 is connected between the collector and emitter of the IGBT 124. The gate electrodes of the IGBTs 123 and 124 are biased by a gate controller 116. The IGBTs 123 and 124 are manufactured according to the processes of FIGS. 2A to 2D. The switch of FIG. 4 is capable of passing a current between the output terminals 121 and 122 in both directions. Namely, the switch controls a direct current as well as an alternating current.
The ON-state voltage V.sub.ON of the bidirectional semiconductor switch of FIG. 3 consisting of MOSFETs is higher than that of the bidirectional semiconductor switch consisting of IGBTs. In addition, the two MOSFETs connected oppositely in series further increase the ON-state voltage. For example, the ON-state voltage of a MOSFET of 500 V in breakdown voltage is about three times higher than that of an IGBT of the same breakdown voltage and chip area. In addition, the ON-state voltage of the semiconductor switch of FIG. 3 is doubled because the MOSFETs are connected oppositely in series.
In FIG. 4, one of the reverse diodes 125 and 126 is connected to a current path in series, and therefore, the ON-state voltage of the reverse diode is added to the ON-state voltage of the IGBTs, to increase the total ON-state voltage. The breakdown voltage and current capacity of each reverse diode must be equal to those of the corresponding IGBT, to increase the cost.